In typical known processes for producing terephthalic acid, p-xylene is oxidized to the product terephthalic acid. P-xylene is continuously or batchwise oxidized in a primary oxidation reactor in the liquid phase in the presence of an oxygen containing gas such as air. P-xylene, an oxidation catalyst, a molecular source of oxygen, and a solvent such as acetic acid are combined in a reactor to produce a crude terephthalic acid composition. A typical oxidation catalyst composition is made by contacting a cobalt compound with a manganese compound, usually also in combination with a promoter such as a bromine compound.
The resulting terephthalic acid product is not very soluble in a solvent such as acetic acid under the reactor operating conditions, and usually crystallizes out of the solvent as a solid to form a suspension. The crude terephthalic acid composition in the primary oxidation reactor is a reaction mixture which contains terephthalic acid solids, a solvent acting as the suspending medium for the solids and containing a small amount of terephthalic acid dissolved therein, catalyst, unreacted p-xylene, incompletely oxidized intermediate oxidation products such as para-tolualdehyde, para-toluic acid, 4-carboxybenzaldehyde (4-CBA), and other organic impurities which may cause discoloration dissolved in the solvent. The crude terephthalic acid composition is discharged from the oxidation zone and generally subjected to a variety of mother liquor exchange, separation, purification, and recovery methods, resulting in recycling back to the oxidation zone the recovered solvent and catalyst composition. It would be desirable to reduce the amount of incompletely oxidized intermediates (“intermediates”). By reducing the amount of intermediates, primarily composed of 4-CBA, one may either improve the yield, reduce the volume of mother liquor containing the intermediates which must be separated from the terephthalic acid product, reduce the amount of intermediates needed in a post oxidation reactor, or all the foregoing.
Other by-products of the liquid phase oxidation which are partially or completely removed from the reaction mixture in the oxidation reactor are the off-gases which include water, solvent, unreacted oxygen and other unreacted gases found in the source of the molecular oxygen gas such as nitrogen and carbon dioxide, and additional a mounts of carbon dioxide and carbon monoxide produced by the catalytic decomposition of the solvent under the oxidation conditions. The off-gases are vented at the overhead of the oxidation reactor to a distillation column or a condenser to separate the solvent from the other off-gases such as water, carbon dioxide, carbon monoxide, nitrogen, methyl bromides, etc. The solvent recovered in the distillation column or condensers is recycled back to the oxidation reactor for further use. The hot uncondensed gases are removed from the distillation column and sent to energy recovery devices, such as turboexpanders and electric generators, or to heat exchangers, or to steam generators, optionally before or after passing through catalytic oxidation or other suitable equipment for neutralizing or removing acidic and corrosive ingredients in the gaseous stream.
The oxidative decomposition of the solvent in the primary oxidation reactor resulting in the generation of carbon dioxide and carbon monoxide gas is referred to as the solvent burn, and results in the loss of solvent. It is desirable to recover and recycle back to the oxidation reactor as much solvent as possible for further use. However, once the solvent is decomposed in the primary oxidation reactor into its constituent gaseous products, such as carbon monoxide and carbon dioxide when acetic acid is the solvent, there no longer exists solvent to recover resulting in the permanent loss of solvent and requiring a fresh source of make-up solvent. Reducing the amount of solvent burn would significantly lower the operating costs in the oxidation zone by allowing a greater amount of solvent to be recovered and recycled back to the oxidation zone and by lowering the amount of fresh make-up feed. However, the reduction in solvent burn should not come at the expense increasing the amount of 4-CBA in the crude mixture, and if possible, it would be desirable to simultaneously reduce the solvent burn and reduce the amount of 4-CBA generated in the crude oxidation mixture.